It is known to treat thermoplastic substrates such as polycarbonate articles, for example, with photocurable acrylic coating compositions as shown in U.S. Pat. Nos. 4,455,205 and 4,491,508 to Olson et al. and in U.S. Pat. No. 4,198,465 to Moore et al., all assigned to the same Assignee as the present invention. Although cured polyacrylate coatings are physically and chemically durable, some polyacrylic coatings often do not satisfy the abrasion resistant standards required in many applications. Further, processes for producing these coating compositions are time and energy consuming. In some instances, the processes are hazardous.
For example, U.S. Pat. Nos. 4,491,508 and 4,455,205 to Olson et al. disclose processes for producing UV curable coating compositions which processes include extended periods of distillation or reflux to remove water and large amounts of water miscible alcohols. According to these disclosures, water miscible alcohol comprises greater than 50% (and in some cases greater than 75%) of an initial mixture of water miscible alcohol, colloidal silica, silyl acrylate and acrylic monomer.
Oxygen present during the strip step minimizes premature polymerization of the acrylic monomer. The addition of a free radical scavenger protects the acrylic monomer, especially under the conditions of vacuum and heat which are called for.
Isopropyl alcohol (IPA) is used to remove water during the solvent strip step. IPA has a low flash point. Introduction of oxygen in amounts greater than 10% above the flash point is dangerous. However, if less than about 5% oxygen is added, the acrylic monomer will begin to polymerize. Introduction of oxygen at such a precise concentration further complicates the process.
A need therefore exists for a method of making a UV curable hardcoat composition which is safe, time and energy efficient, and has good kettle utilization in terms of the volume ratio of initial reactants to resultant products.